The present invention generally relates to a system and method for distorting a signal and more particularly relates to a system and method for mapping a distortion characteristic to a signal.
In the area of digital signal processing, particularly in the area of music signals, it is desirable to add a predetermined distortion to a signal. In the creating of digital models of old analog equipment, it is necessary to copy the particular equipment""s distortion characteristics. The prior art has sought to accomplish this copying by applying transfer curves either derived empirically by trial and error or determined by a submodel of the active component creating the distortion. In a typical analog system, there may be many devices contributing to the distorted output therefore requiring many submodel to achieve the desired distortion characteristics.
U.S. Pat. No. 5,789,689 to Doidic discloses an electric guitar amplifier which utilizes a digital signal processor to produce vacuum-tube-like distortion. U.S. Pat. No. 5.929,795 to Wang digitally processes a signal for reduced distortion and frequency deviation. The digital processing involves increasing the sampling frequency of a digital signal prior to a non-linear operating stage. The processed signal is then passed through a low pass filter prior to being down-converted to the initial sampling frequency.
It is an object of the present invention to provide a system and a method for distorting an input signal. It is desirable to obtain distorted audio signals in the audio or visual recording studio to simulate various parts of the recording signal chain such as recording machines and outboard effect devices. The present invention has eliminated the trial and error and modeling techniques of the prior art as described above, and uses data obtained from the output spectra of the modeled device to create a transfer curve directly. The output spectra can be obtained readily from an analog unit under test through Fast Fourier Transform (FFT) analysis.
The present invention employs a mapping function in order to apply a known distortion to an input signal. The transfer curve of the present invention is defined by: y=A1*sin(arcsin(x))+A2*cos(2*arcsin(x))+A3*sin(3*arcsin(x))+A4*cos(4*arcsin(x))+An*sin(n*arcsin(x))+A(n+1)*cos((n+1)*arcsin(x)), where x is the input signal, y is the output signal, A is the amplitude of the harmonic level for a given harmonic of the distortion, 1 being the fundamental, n is an odd number and (n+1) represents the highest harmonic of interest.